An internal combustion engine generally comprises a plurality of cylinders, each having at least one intake valve and at least one exhaust valve. The intake valves are fluidly connected with an intake system for feeding ambient air into the cylinders, and the exhaust valves are fluidly connected with an exhaust system for discharging exhaust gas from the cylinders to the environment. The intake system generally comprises an intake manifold connected to the ambient air, and a plurality of intake conduits that lead from the intake manifold to each intake valve.
In order to reduce polluting emission, most internal combustion engines are provided with an EGR system, which is arranged for routing back exhaust gas from the exhaust system to the intake system. One of the main benefits of the EGR system is the reduction of NOx emission. However, it has been experimentally found that, during the start phase of an IC engine with EGR, namely during the first combustion cycles after the engine start, the amount of NOx emitted in the environment is higher than expected. This NOx peak is due to several adverse conditions that are typical of the start phase of the engine.
A first adverse condition is that the EGR circuit is substantially empty, since the IC engine has been inactive for a certain stop time without thereby producing exhaust gas. Another adverse condition is that the internal combustion engine takes several combustion cycles before filling the EGR circuit with exhaust gas. A further adverse condition is that the EGR valve is quite far from the engine cylinders, so that the exhaust gas reaches the latter with a certain delay. Another adverse condition is that, when the internal combustion engine is off, the intake manifold contains a lot of oxygen (O2) that, in combination with the late EGR recirculation, increases the production of NOx.
The above mentioned drawback is particularly important with regard to the vehicles in which a start and stop (S&S) functionality is implemented in the Engine Control Unit (ECU). The S&S functionality provides for the ECU to automatically shut off the engine when the vehicle speed is zero, avoiding in this way the fuel consumption during idle condition, that is the worst efficient engine operating condition, and to automatically restarting the engine when the vehicle is commanded to move, namely in response of the crank-in after a stop time. Because of the many start phases, an IC engine with EGR and S&S functionality may globally emit more NO than an IC engine with EGR but without S&S functionality, and eventually exceed the limit set by the strictest antipollution standards. Experimental data confirm that, in an IC engine with EGR, the S&S functionality causes about 5% NOx increase along the New European Driving Cycle (NEDC).
In view of the foregoing at least one object is to solve, or at least to positively reduce, the above mentioned drawback with a simple, rational and cheap solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.